1. Field of the Invention
The present invention relates to a fuel injection pump for an internal combustion engine (hereinafter called xe2x80x9cenginexe2x80x9d) in which fuel is supplied from a feed pump to an accommodation chamber for well lubricating sliding contact portions of component parts accommodated therein.
2. Description of the Prior Art
A conventional fuel injection pump for a diesel engine has a cam for driving a plunger. In this pump, fuel is sucked and pressurized in a pressure chamber by reciprocating movement of the plunger axially slidable in a cylinder. A rotating movement of a drive shaft to be driven by an engine is converted to the reciprocating movement of the plunger inside the cylinder via the cam connected with the drive shaft and a cam ring disposed between the cam and the plunger. The plunger, the cam and the cam ring are disposed in an accommodation chamber.
To improve engine output and fuel consumption and to reduce emission such as NOx and black smoke to be exhausted from the engine, higher fuel injection pressure has been recently demanded.
To secure the higher fuel injection pressure, it is necessary to increase pressure of fuel to be pressurized by and discharged from the fuel injection pump, so higher load is applied to the fuel injection pump. Typically, sliding contact portions of an outer surface of the cam and an inner circumference of the cam ring receive larger forces. To reduce frictional seizure and frictional wear of the sliding contact portions, the conventional pump has a bush interposed between the cam and the cam ring. However, since the force applied to the bush is very high, life time of the bush is shorter and, at worst, frictional seizure of the sliding contact portions tends to occur.
As one of the solutions, the pump may have a larger bush whose sliding contact area is larger so that force applied to a unit area of the bush is smaller. However, the larger bush causes a body of the fuel injection pump larger, which does not meet a recent demand in that a compact and light weight pump is required.
As another one of the solutions, the outer circumference of the cam or the inner circumference of the bush may have an oil groove for promoting lubrication of oil on the sliding contact portions of the cam and the bush. However, the oil groove has to be positioned to a limited area of the cam or the bush in order not to sacrifice power transmission functions of the cam and bush so that their designing freedom is narrowed.
Further, as shown in FIG. 14, a bush 100 may have a recess 101. An inner circumferential surface 100a of the bush 100 is formed axially in round shape. Diameter of the inner surface 100a of the bush 100 is larger from both axial ends thereof toward a center thereof so that the both axial ends are always in contact with an outer circumference 210a of a cam 210. Fuel once entered the recess 101 tends to be stagnant in the recess 101. Therefore, temperature of fuel in the recess 101 is prone to increase due to heat generated by the sliding contact between the cam 200 and the bush 100 so that deformation or frictional seizure of the cam 210 and the bush 100 is likely to occur. Further, pressure of fuel in the recess 101 tends to decrease, if the recess 101 is too large, so that adequate formation of oil film for lubricating the sliding contact portions of the cam 200 and the bush 100 is not sufficiently secured.
As mentioned above, it is important not only to form the adequate formation of the oil film between the sliding contact portions, but also to radiate heat of the sliding contact portions in the accommodation chamber. For this purpose, the conventional fuel injection pump has a feed pump driven also by the drive shaft for supplying fuel sucked from a fuel tank to the pressure chamber. The feed pump also supplies a part of the fuel to the accommodation chamber where the cam connected to the drive shaft, the cam ring, the bush and the plunger are accommodated.
Temperature of the fuel supplied to the accommodation chamber increases due to heat generated by the sliding contact portions. Therefore, the conventional fuel injection pump has a circulation flow passage through which the accommodation chamber communicates with a fuel tank, so the fuel discharged from the feed pump is returned to the fuel tank via the accommodation chamber through the circulation flow passage, which serves to lubricate the sliding contact portions and to cool the housing in which the accommodation chamber is formed.
Further, to prevent the pressurized fuel in the accommodation chamber from leaking through a clearance between the housing and the drive shaft to an outside of the fuel injection pump, an oil seal is disposed between the housing and the drive shaft. The oil seal is partly deformed radially to ensure sealing effect by pressure of the fuel fed thereto from the accommodation chamber.
However, if the fuel in the circulation flow passage is blocked by some reasons, the fuel in the accommodation chamber is not returned to the fuel tank so that pressure of fuel in the accommodation chamber increase. As a result, higher pressure of the fuel in the accommodation chamber is applied to the oil seal, causing the oil seal to be damaged.
An object of the present invention is to provide a compact fuel injection pump at less manufacturing cost.
Another object of the present invention is to provide a fuel injection pump in which fuel is forcibly supplied to sliding contact portions to form an oil film for lubrication in a minute space therebetween so that frictional seizure of the sliding contact portions is unlikely to occur and life time thereof is longer.
A further object of the present invention is to provide a fuel injection pump in which fuel for lubrication easily circulates through sliding contact portions within an accommodation chamber so that heat generated by the sliding contact portions is effectively radiated.
A still further object of the present invention is to provide a fuel injection pump in which an oil seal is hardly damaged.
To achieve the above object or objects, a fuel injection pump has a drive shaft, a cam formed eccentrically and integrally with the drive shaft, a cam ring, a bush whose outer circumference is fixed to inner circumference of the cam ring and whose inner circumference is slidably fitted to outer circumference of the cam shaft so that sliding contact portions of the cam and the bush are formed, a housing provided with a cylindrical bore, a plunger axially and reciprocatingly movable in the cylindrical bore via the cam, the bush and the cam ring by the drive shaft, so that axial end of the plunger is in slidable contact with outer circumference of the cam ring and another axial end thereof and the cylindrical bore form a pressure chamber, an accommodation chamber formed in the housing for accommodating the plunger, the bush, the cam ring and the cam, a feed pump whose outlet side communicates with the pressure chamber for supplying fuel to the pressure chamber according to rotation of the drive shaft, and a communication path through which the outlet side of the feed pump also communicates with the accommodation chamber for supplying a part of the fuel discharged from the feed pump to the accommodation chamber.
In the fuel injection pump mentioned above, an end of the communication path on a side of the accommodation chamber is opened to face an axial end of a minute space between the sliding contact portions of the cam and the bush when the cam is at a given angular position. With this construction, fuel flowing via the communication path to the accommodation chamber is sprayed axially deep into the minute space between the sliding contact portions from the axial end thereof. Accordingly, the oil film for lubrication is adequately formed on the sliding contact portions so that the frictional seizure of the contact sliding portions hardly occurs. Since only position of the end of the communication path on a side of the accommodation chamber is defined, shapes of the cam and the bush can be freely designed, which achieves a compact fuel injection pump at less manufacturing cost.
In the fuel injection pump, as an alternative, the cam ring may be provided with a recess formed on the outer circumference thereof and a guide hole through which the recess communicates with a minute space between the sliding contact portions of the cam and the bush.
In this case, the end of the communication path on a side of the accommodation chamber may be opened to any position of an inner wall of the accommodation chamber. Even if the axial end of the communication path is positioned radially outside the cam ring, fuel is supplied to the sliding contact portions through the guide hole.
If the end of the communication path on a side of the accommodation chamber is positioned to face the axial end of a minute space between the sliding contact portions of the cam and the bush when the cam is at a given angular position, the sliding contact portions is filed not only with the fuel sprayed axially from the communication path to the accommodating chamber but also with the fuel fed through the recess and the guide hole.
The recess may be a groove extending axially along an axis of the drive shaft from an end of the outer circumference thereof on a side of the communication path toward another end of the outer circumference thereof on a side opposite to the communication path. In this case, the end of the communication path on a side of the accommodation chamber may be positioned to face the axial end of the groove on a side of the communication path when the cam is at a given angular position. With this construction, fuel flowing via the communication path to the accommodation chamber is sprayed to the axial end of the groove so that fuel is forcibly introduced through the groove and the guide hole to the sliding contact portions. Accordingly, the oil film for lubrication is adequately formed on the sliding contact portions, which reduces the frictional seizure of the contact sliding portions.
The groove may come to a dead-end before reaching the end of the outer circumference of the cam ring on a side opposite to the communication path. In this case, the fuel flowing along the groove hits against the dead end so that the fuel deflected is easily introduced into the guide hole.
It is preferable that one end of the guide hole communicates with the recess or groove and another end thereof communicates with the minute space between the sliding contact portions of the cam and the bush in a vicinity of an axial center of the cam ring. Though the fuel normally enters the minute gap between the sliding contact portions from the opposite axial ends thereof so that formation of the oil film for lubrication in a vicinity of the axial center of the cam ring tends to be insufficient, the groove and the guide hole make it possible that entire axial surfaces of the sliding contact portions are covered uniformly with oil film for lubrication.
As another alternative, at least one of the inner circumference of the bush and the outer circumference of the cam may gradually protrude radially toward the other one of the inner circumference of the bush and the outer circumference of the cam from the axial opposite ends to the axial center thereof.
Only one of the inner circumference of the bush and the outer circumference of the cam may gradually protrude radially toward the other one of the inner circumference of the bush and the outer circumference of the cam that has axially flat surface.
It is preferable that the one of the inner circumference of the bush and the outer circumference of the cam is formed axially in a curve. Preferably, a protruding amount t of the one of the inner circumference of the bush and the outer circumference of the cam falls within a range of 0 mm less than txe2x89xa60.01 mm.
Further, not only the inner circumference of the bush may protrude radially inward toward the outer circumference of the cam but also the outer circumference of the cam may protrude radially outward toward the inner circumference of the bush. In this case, each of the inner circumference of the bush and the outer circumference of the cam is formed axially in a curve and, preferably, a sum t of a protruding amount of the inner circumference of the bush and a protruding amount of the outer circumference of the cam falls within a range of 0 mm less than txe2x89xa60.01 mm.
With the fuel injection pump having at least one of the inner circumference of the bush and the outer circumference of the cam that is formed axially in a curve, even if the position of the end of the communication path on a side of the accommodation chamber is not limited to a given point of the inner wall of the accommodation chamber, the fuel can easily enter the sliding contact portions from the opposite axial ends of the bush or the cam toward the axial center thereof so that the oil film for lubrication is easily formed between the inner circumference of the bush and the outer circumference of the cam.
Further, the fuel heated due to sliding contact between the cam and the bush can be easily ejected to an outside and does not remain in the minute space between the sliding contact portions so that the bush and the cam are hardly damaged by heat, contrary to the prior art.
As a further alternative, it is preferable that the fuel injection pump has a fuel return passage through which the accommodation chamber communicates with the inlet side of the feed pump and a check valve disposed in the fuel return passage for permitting fuel flow from the accommodation chamber to the inlet side of the feed pump when pressure of the accommodation chamber exceeds a given value.
If a fuel circulation passage for ejecting the fuel from the accommodating chamber is blocked or stuffed with foreign material containing in the fuel, fuel pressure in the accommodation chamber increases. As a result, higher pressure of the accommodation chamber is applied to an oil seal for sealing the drive shaft and the housing. In case that the pressure of fuel applied to the seal exceeds endurable pressure of the oil seal, the oil seal tends to be damaged.
However, according to the fuel injection pump mentioned above, the check valve disposed in the fuel return passage is opened before the pressure of accommodation chamber exceeds the endurable pressure of the oil seal, so the fuel in the accommodation chamber is returned to the inlet side of the feed pump without increasing abnormally the pressure of the accommodation chamber.
It is preferable that a temperature sensor is installed in the accommodation chamber for detecting temperature of fuel therein to control operation of the fuel injection pump and, when the temperature detected by the temperature sensor exceeds a given valve, an operation of the fuel injection pump stops.
When the check valve continuously opens, the fuel is circulated between the feed pump and the accommodation chamber. Due to the heat generated from the sliding contact portions, temperature of the fuel circulating between the feed pump and the accommodation chamber increases to an extent that the frictional seizure of the sliding contact portions may occur. However, since the temperature of the accommodation chamber is detected and, based on the detected temperature, the operation of the engine stops before the frictional seizure of the sliding contact portions occurs.